efi_loader: populate console handles in system table

The fields ConsoleInHandle, ConsoleOutHandle, ConsoleErrHandle must point
to the handles with the respective console protocols. Failure to do so
leads to an error in the EFI Shell:

    No SimpleTextInputEx was found. CTRL-based features are not usable.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>